(i) Field of the invention
This invention relates to spray elements for use in a rotating disc filter assembly.
(ii) Description of the Prior Art
Rotating disc filters are used in many industrial filtration processes. In the pulp and paper industry, such filters are used for pulp filtration and thickening in a so-called thermo-mechanical pulping (T.M.P.) process which is an important treatment before final pulp processing in paper machines which produce a paper product.
Filtration and thickening of pulp is normally accomplished using rotating disc filters to separate the pulp from so-called cloudy white water solution. In this process, pulp fibres attach themselves to both faces of each rotating disc while the discs are submerged in the cloudy white water solution. Then the discs are simply lifted out above the surface by rotation. At a certain predetermined point of disc rotation, thickened pulp is mechanically detached (often by a method called "sluicing") from the two faces of the disc and discharged to a chute for further processing.
It is imperative that the disc faces be thoroughly cleaned with fresh, clean water after removal of the pulp and prior to subsequent reimmersion of the discs into the cloudy white water to complete the rotational cycle.
Improper or inadequate cleaning of the disc faces results in build-up of pulp "lumps" which leads to loss of efficacy caused by plugging of the disc filter itself.
These are two basic types of washing system commonly being used:
(a) "stationary" cleaning showers; and PA1 (b) "oscillating" cleaning showers.
Stationary cleaning showers are simply straight pipes supplied with wash water from a main header, inserted between the rotating discs of the filter and having multiple-spray nozzles pointing towards the disc's surface. Water jets discharging from the nozzles continuously wash the surface of the rotating discs. Such an arrangement is disclosed in U.S. Pat. No. 3,532,220. This particular system offers good cleaning coverage of the disc surface, however, its multiple-nozzle spray arrangement consumes unacceptably high quantities of highly pressurized cleaning water.
"Oscillating" type cleaning showers have normally one or two nozzles located at the end of a supply pipe connected to a similar main supply header. The supply header is mechanically set in oscillating motion causing "pendulum" like motion of the supply pipes between adjacent discs. Consequently, the nozzles located at the end of these supply pipes oscillate between the outer edge of the discs and the main shaft on which the discs are mounted. Such a system is disclosed in a technical brochure on the Impco Disc Filter of Ingersoll- Rand Ltd. in FIGS. 3-2A and 3-3A on pages identified by the numerals BK46 1072 and BK47 1069 respectively. This system consumes moderate quantities of wash water but leaves large areas of the disc surface uncleaned during each rotational cycle leading to eventual pulp build-up on the disc surface and subsequent plugging.